I've changed my flight plenty of times but the one time I've ran into a issue, was when an international flight I had originally booked wasn't in the typical blue basic, blue and blue extra. I don't remember what the names of the other fare classes were but for blue, on the international flight I had booked (JFK-CTG) it was blue plus. This flight was also booked using the change function to take advantage of the expired credit. I tried to change this flight to a different one but couldn't. I chatted with a jetblue rep and he mentioned that it was due to the different fare classes (blue plus) not being available on the new flight. Only way to change to a new flight was to cancel and rebook. The chat rep was able to reinstate the expired funds and used that to book a new flight for me.

This could be why you're not able to switch to an international flight. If it's the case, talk to a jetblue rep. If they say that the only way to change is to cancel and rebook, tell them about the expiring/expired credit that's attached to the domestic one you're trying to change and they should be able to reinstate it before booking your new flight.